In conventional movie making etc., in order to obtain special image effects, an image has been shot with a film camera allowing a shooting speed, that is, the number of frames per second to be altered. For example, shooting is performed at a speed higher than an ordinary one and the reproduction therefor is performed at the ordinary speed. In this case, a reproduced image provides a viewer with a high-speed motion-picture like a water drop falling down into a surface of water so that he or she can finely observe it with ease. In another case, shooting is performed at a speed lower than an ordinary one and the reproduction therefor is performed at the ordinary speed. In this case, a reproduced image provides a viewer with a realistic picture like a martial art scene or a picture with a feeling of speediness like a car chase scene.
Although digitalization of image shooting, edition, broadcasting and the like of a program has been developed in TV program making or the like, digitalization of movie making or the like has also been developed with an improvement in picture quality and a reduction in equipment costs owing to a progress in digital technologies.
By the way, if shooting an image is performed using a video camera in the digitalized movie making or the like, varying an interval of output from an image pick-up device, for example, a Charge Coupled Device (CCD), allows the image shooting speed, that is, the number of frames per unit time as described above to be altered. However, a tolerable charge storage quantity of each pixel of a CCD is predefined in accordance with a structure etc. of the image pick-up device. If charge is stored in excess of the tolerable charge storage quantity of the CCD, a trouble such as blooming occurs. Therefore, during a low-speed shooting (when the number of frames per unit time is made to be smaller than that of ordinary shooting), in order to avoid storage of charge in excess of the tolerable charge storage quantity of the CDD, it is necessary to narrow an iris down mechanically or electronically to decrease a quantity of light incident to the CCD or to reduce a shutter-open period of a mechanical or electronic shutter.
FIGS. 1A-1C show modeling of a charge storage quantity in a CCD during the ordinary shooting and the low-speed shooting. FIG. 1A indicates a case of the ordinary shooting wherein “QM” indicates a tolerable charge storage quantity of each pixel, “QA” indicates a charge storage quantity of a shot image, and “QN” indicates a charge storage quantity due to noise(s). Note that the charge storage quantity of a shot image varies with a quantity of light made incident upon the CCD per unit time.
FIG. 1B indicates the case of low-speed shooting. During low-speed shooting, if, for example, an incident light quantity is the same as that for ordinary image shooting and also a shooting speed is ⅓ times the ordinary shooting speed, a threefold charge storage period of the CCD increases. Therefore, the charge storage quantity of a shot image becomes “3QA” and that owing to noise becomes “3QN”, resulting in the charge storage quantity of the shot image and that owing to noise exceeding the tolerable charge storage quantity. This causes the CCD to encounter a trouble such as the blooming described above.
FIG. 1C shows a case of low-speed shooting where an iris or the like is used to reduce a quantity of light made incident to a CCD so that the sum of a charge storage quantity of a shot image and that owing to noise may not exceed the tolerable charge storage quantity. For example, if the shooting speed is reduced to ⅓ and also the iris is narrowed to reduce an incident light quantity to ⅓ so that the same charge storage quantity may be provided, a threefold charge storage period of the CCD increases, resulting in a charge storage quantity owing to noise of “3QN” hence that of the shot image of, for example, “QB”. Thus, the reduced charge storage quantity of the shot image prevents the sum of the charge storage quantity of the shot image and that owing to noise from exceeding the tolerable charge storage quantity “QM”. However, a ratio of the charge storage quantity owing to noise with respect to that of the shot image increases. This causes the shot image to be deteriorated in S/N ratio, resulting in noisier picture.
Further, if the low-speed shooting is performed with a prolonged charge storage period in a CCD as described above, a shot image based on a main line image signal having a desired frame rate obtained by the low-speed shooting has a longer image update interval. Therefore, if the main line image signal is used as a monitor image signal to monitor images, the update interval of a shot image displayed on an image display apparatus such as an electronic viewfinder is prolonged. In this case, if, for example, a camera operator adjusts a shooting direction of camera to shoot a speedily moving subject at the center on a screen of the electronic viewfinder with the operator referring to the image displayed on the electronic viewfinder, an actual position of the subject comes ahead a displayed position, so that no subject can be shot at a desired proper position.